Silica refers to silicon dioxide (SiO2) or refers generally to substances each of which is mainly formed of silicon dioxide. Silica is produced in large amounts as a main component of each of naturally occurring quartzite and silica sand, which, however, contain large amounts of impurities. Production of a semiconductor or a solar cell requires silica, but silica for such use has a high purity and is expensive.
Rice straw and chaff that are discharged as agricultural waste in large amounts are mostly discarded unutilized. In view of this, attempts have been made to recover silica contained in each of the rice straw and the chaff.
As the attempts to recover silica, the following patent applications have been reported.
In Patent Literature 1, there is a disclosure of a “method of producing high-purity silica from chaff, including the steps of: (a) treating chaff with a hydrofluoric acid HF solution to extract a silicon compound contained in the chaff as hexafluorosilicic acid H2SiF6; (b) adding ammonia to the extract to convert the hexafluorosilicic acid to ammonium hexafluorosilicate (NH4)2SiF6 to be decomposed into ammonium fluoride NH4F and silica SiO2; (c) separating silica precipitated as amorphous powder by filtration, followed by water washing and drying, to obtain high-purity silica; and (d) adding a strong acid to a filtrate containing the ammonium fluoride to generate hydrogen fluoride, which is subjected to cyclic use in the step (a).”
The recovery method disclosed in Patent Literature 1, in which the treatment with hydrofluoric acid is performed, is obviously different from a recovery method of the present application.
In Patent Literature 2, there is a disclosure of a “method of producing amorphous silica from chaff, characterized by: treating chaff with an acetic acid-containing solution to separate an alkali component contained in the chaff from the chaff through dissolution in the acetic acid-containing solution; washing and drying the chaff from which the alkali component has been separated; carbonizing the dried chaff through thermal decomposition to form chaff charcoal; and calcining the chaff charcoal to produce high-purity silica.”
However, the recovery method disclosed in Patent Literature 2, in which the acetic acid treatment is performed, is obviously different from the recovery method of the present application.
In Patent Literature 3, there is a disclosure of a “production method for finely powdered silica, characterized by: dissolving precipitated silica derived from a natural product in liquefied carbon to prepare a silica-containing solution; compressing the silica-containing solution; and then rapidly decompressing the compressed silica-containing solution to cause expansion, to thereby evaporate carbon dioxide in the solution and recover finely powdered silica.”
However, the recovery method disclosed in Patent Literature 3, in which the dissolution treatment with liquefied carbon and the like are performed, is obviously different from the recovery method of the present application.
In Patent Literature 4, there is a disclosure of a “method for separating one or more components of interest from a sample comprising the steps of: (a) filtering a sample containing particulates and soluble materials through silica filter media whose surface active groups have reacted with one or more silanes; (b) simultaneously capturing particulates and binding a soluble component of interest to the silica filter media; and (c) eluting the component of interest from the silica filter media.”
However, the recovery method disclosed in Patent Literature 4, in which the sample is filtered through the filter media having surfaces treated with silanes, is obviously different from the recovery method of the present application.
In recent years, there have been proposed various methods of treating and recycling plastic waste, and further, parts thereof have been practically employed. As a potent one of such methods of treating plastic waste, there has been proposed an apparatus and method involving gasifying the plastic waste by heating chips of the plastic waste in the presence of a decomposition catalyst of titanium oxide known as a photocatalyst (see Patent Literatures 5 and 6).
In addition, various catalysts to be used in decomposition treatment of the plastic waste have been investigated (Patent Literatures 7 to 12).